A typical gas discharge lamp utilizes an electronic ballast to convert AC line voltage to a high frequency current capable of powering the gas discharge lamp. The main component of such an electronic ballast is a resonant DC/AC inverter, which is typically a series resonant DC/AC inverter driven by a control integrated circuit (IC) chip.
A single electronic ballast may power a plurality of lamps placed in a fixture including the electronic ballast. When replacing one or more lamps, or when one or more lamps are damaged, the resonant DC/AC inverter of the electronic ballast should operate without a load being present. During the startup period of a lamp, that is, when the lamp first starts, and particularly during instant starting, that is, without starting the lamp without first preheating the lamp, the electronic ballast operates in a no load condition for some interval of time. Electronic ballasts should output a high frequency starting voltage, for example, up to 1000V rms (root mean squared), to instant start one or more fluorescent lamps. For high intensity discharge (HID) lamps, the electronic ballast should output an even higher starting voltage. A higher output voltage by the electronic ballast during the startup period, including instant starting, is required to compensate for voltage losses in cables or wires connecting remote lamps to the electronic ballast. Industrial wires and connectors used in lighting application are typically rated for a maximum of 600V rms continuously applied. In a no load condition, the output voltage of the electronic ballast may exceed this number.
Excepting voltage stress on components, another obstacle for continuous no load operation is a higher power loss concentrated in the transistors of the inverter and in a resonant inductor. Even total ballast power loss typically does not exceed the same under a full load. U.S. Pat. No. 7,372,215 issued to Sekine et al. teaches avoiding an open circuit mode in a multi-lamp ballast by sensing the conductivity of a lamp via lamp filaments, and shutting down the inverter when no load is connected. The system taught by the '215 patent requires additional wires between each lamp and the ballast. U.S. Pat. Nos. 6,952,085 and 6,975,076, both issued to Nerone, disclose a control block for pulse inverter operation during starting as well as open circuit. These blocks include a pulse-width modulated (PWM) controller IC, such as a UC3861 from Texas Instruments recommended by the disclosures, a control transformer, and other active and passive components. The systems taught by the '085 and '076 patents detect open circuit conditions by having a resonant tank clamping circuit activated. U.S. Pat. No. 6,326,740 issued to Chang et al. discloses a no load voltage fed resonant inverter in a multiple-lamp ballast having over-voltage control with a flip-flop. This over-voltage control provides an on/off pulse inverter operation via an over-voltage feed back loop. The flip-flop provides a stable on/off inverter operation in sleep mode. However, the systems taught by the '085 and '076 patents feature an indirect detection of no load conditions via an output voltage sense features signal delay, which requires use of many complex surrounding circuits and ICs. A similar rms voltage limiting feature during lamp startup is used in an HID ballast according to U.S. Pat. No. 7,119,494 issued to Hui et al.